Well treatment fluids often consist of gels formed from soluble dispersible gums mixed with water. Such fluids are particularly useful in the control of fluid loss in gas or oil well drilling operations, as proppant carriers and as fluid loss control agents in well completion and workover operations.
Such applications require the gels to be operational at elevated temperatures. Unfortunately when gels of water soluble gums are exposed to elevated temperatures for any extended length of time, they lose their viscosity in part or in whole and therefore become less effective or completely ineffective. Dissolved oxygen is the major cause of an oxidative free radical polymer breakdown which leads the reactivity of oxygen to increase exponentially with temperature and pressure increases and become significant at high temperatures. Thus, well treatment fluids generally deteriorate rapidly at elevated temperatures. To prevent premature viscosity degradation, oxidation inhibitors or free radical scavengers are a necessary component of gels used in hot wells.
Oxygen scavengers are reducing agents in that they remove dissolved oxygen from water by reducing molecular oxygen to compounds in which oxygen appears in the lower −2 oxidation state. The reduced oxygen then combines with an acceptor atom, molecule or ion to form an oxygen-containing compound. To be suitable as an oxygen scavenger, the reducing agent must have an exothermic heat of reaction with oxygen and have reasonable reactivity at lower temperatures.
In oil field operations, the oxygen scavenger has to remove available oxygen at low temperatures before it can damage vulnerable polysaccharide gums at higher temperatures. Oil field applicable oxygen scavengers must have a limited lifetime. They should be consumed in the course of a treatment so that they do not interfere with the after treatment gel breaking process
Chemical incompatibility between reducing agents and crosslinkers further creates another product choice limitation. Since all oxygen scavengers are reducing compounds, they are electron donors. Electron donors are Lewis bases capable of chelating metals which makes them unsuitable for application in metal crosslinked gels. Based on the above limitations, most common antioxidants have been rejected from oil field applications.
Compositions for the reduction of thermal degradation of aqueous gels by the addition of a gel stabilizer have been reported in U.S. Pat. No. 5,362,408 wherein the stabilizer contains an oxime as electron donor compound. Such donor compounds were shown as being capable of stabilizing gels at temperatures as high as 150° C. (302° F.). A need exists for well treatment fluids of gels capable of being stabilized at temperatures as high as 204.5° C. (400° F.) as deeper wells are explored.